Power levels and densities are greatly increasing for the next generation of high performance integrated circuit (IC) electronic devices, such as solid state lasers, power amplifiers, flip chip designs, and phase array antenna elements. As densities increase, device sizes are reduced and thermal management bottlenecks and heat dissipation challenges become a limiting factor in performance gains. High levels of heat generated by hundreds of millions of transistors within a few square centimeters can cause reliability issues if the heat is not dissipated effectively. These reliability issues include, for example, fractures, delamination, melting, corrosion and even combustion.
There is a demanding need for substantially more efficient thermal control systems to meet these challenges. Initially, heat sinks in direct contact with high heat generating IC components were used to dissipate heat. However, even nominally flat surfaces of heat sinks and ICs still appear rough at a microscopic level. This means that there are air gaps between the surfaces that have a high thermal resistance. Thermal interface materials such as greases and conductive particle-filled epoxies have been developed to address this problem. More recently, carbon nanotube arrays (CNTs) have been developed as a more efficient TIM.
R&D efforts have largely been focused on improving bulk thermal conductivity of TIMs employed in fabricating the devices as well as the thermal conductor links and heat sinks used in the thermal management system between devices. While significant improvements in material bulk conductivity have been made and theoretical analyses predict conductivities greater than 20× copper are possible with certain nanomaterials, much less effort and success has been realized in reducing thermal interfacial or contact resistance, which is now becoming the limiting factor to performance gains.
Thermal interfacial resistance is the serial combination of the bulk conductivity of the material between two surfaces and the contact resistances at each material interface. The thermal interfacial resistance governs thermal conductance and the overall thermal control efficiency. Due to microscopic asperities between mating surfaces contact areas are reduced, increasing thermal contact resistance. Reduction in contact resistances can lead to lower device operating temperatures, improved performance, and reliability. Additions of high conductivity nanoparticles to the gaps/void areas in boundary regions increase their heat exchange area and can significantly reduce thermal contact resistance.
Materials previously used for reducing thermal contact resistances have included: filled greases, solders and polymer composite materials. Thermal grease formulations do not meet advanced thermal conductance requirements and are subject to wicking and leakage of fluid carrier. Solder and polymer adhesive based interface agents are subject to debonding due to large thermal stresses caused by mismatch in adherend thermal expansion properties. CNTs have been proposed as substances with high thermal conductivity and low thermal impedence, however; they require expensive manufacturing facilities and high synthesis temperatures that can damage electronic components. Also, the presence of impurities, the existence of voids and the growth conditions of CNTs result in large uncertainties in their performance as TIMs. CNTs that are manufactured separately then inserted between the electronic device and its heat sink have the disadvantage of somewhat higher thermal interface resistance since the individual CNT are not directly attached to the growth surface.
Thus, a need exists for an improved thermal interface material with reduced thermal contact resistance that can be readily manufactured and can be reliably inserted in heat flow path and used to cool high power density electronic devices.